1. Field of the Invention
The present invention relates to an electronic balance provided with a Roberval mechanism. The present invention can be applied to the so-called electronic balance provided with a balance mechanism and also to the so-called electronic scale provided with various load sensors without having a balance mechanism.
2. Description of the Prior Art
In the case of many electronic balances and electronic scales, a weighing dish for mounting a load to be measured is supported by a Roberval mechanism (also referred to as a parallel guide) in order to control the movement of the weighing dish. The Roberval mechanism has a structure in which a movable pillar is supported by an upper and a lower beams parallel with each other for a fixed pillar fixed to or integrated with the frame of a balance mechanism or scale mechanism. Both ends of each beam are respectively connected to the fixed pillar or movable pillar through an elastic fulcrum. A weighing dish is supported by the movable pillar. Then, a load working on the weighing dish is transmitted to an electrical load detecting section through a movable pillar or, moreover, through a lever. The electrical load detecting section includes a shift sensor and an electromagnetic-force generator feedback-controlled by using an output of the sensor as a detected value in the case of an electromagnetic-force-equalizing electronic balance. Further, the section includes a vibration chord and its exciting section in the case of a chord-vibrating scale.
As the above Roberval mechanism, the following Roberval mechanisms are known which have a structure obtained by assembling a fixed pillar, a movable pillar, and an upper and a lower beam as members independent of each other as disclosed in the official gazette of Japanese Unexamined Utility Model Publication No. Sho 63-35924 (1988) and moreover, an integral structure obtained by boring one flat base material as disclosed in the official gazette of Japanese Unexamined Patent Publication No. Sho 63-277936 (1988).
The above Roberval mechanisms respectively prevent a weighing dish from overturning or tilting and moreover, have a function for eliminating an error due to a one-sided load on the weighing dish, that is, a four-corner error (one-sided error).
The four-corner-error eliminating function of the Roberval mechanism is not effectuated before the parallelism of an upper and a lower beam is strictly adjusted. In other words, before elastic fulcrum portions provided for both ends of an upper and a lower beam are adjusted so that vertical intervals of the fulcrum portions coincide with each other. Generally, the accuracy of the parallelism ranges between 0.1 and 10 xcexcm though it depends on an allowable four-corner error (balance accuracy). Therefore, it is difficult to meet the accuracy of parallelism in accordance with the machining accuracy of a part and adjustment while actually changing load mounting positions on a weighing dish after assembling, that is, the so-called four-corner-error adjustment is necessary.
The four-corner-error adjustment is performed by adjusting a one-sided error in the longitudinal direction of each beam of the Roberval mechanism, that is, the axial direction (hereafter referred to as longitudinal direction) and the direction orthogonal to the longitudinal direction (hereafter referred to as crosswise direction) while changing load mounting positions on a weighing dish. Therefore, in the case of a Roberval mechanism of an integral structure, portions corresponding to front, rear, right, and left are removed from a part of elastic fulcrum portions at both ends of an upper and a lower beam respectively or as disclosed in the official gazette of Japanese Unexamined Utility Model Publication No. Sho-35924 (1988), the adjustment mechanism of a corresponding portion is operated in the case of a Roberval mechanism provided with an adjustment mechanism for inching the position of a fixed portion to a fixed pillar of each elastic fulcrum portion.
Among the above Roberval mechanisms, the rigidity in the crosswise direction is lower than that in the longitudinal direction and thereby, a one-sided error easily occurs in the crosswise direction because the integral-structure Roberval mechanism obtained by boring a flat base material has a small crosswise-directional dimension. Therefore, the above integral-structure Roberval mechanism has a problem that it is difficult to correspond to a large weight or large dish.
On the other hand, an assembling-type Roberval mechanism has a problem that the adjustment result of a one-sided error in the longitudinal direction influences that of a one-sided error in the crosswise directions and vice versa and thus, adjustment of the errors is difficult.
The present inventor has proposed an electronic balance capable of solving the above problems and preventing a one-sided error from occurring in the crosswise direction even when using a Roberval mechanism having a small rigidity in the crosswise direction and simplifying the adjustment of a four-corner error of the mechanism compared to the conventional case (refer to the official gazette of Japanese Unexamined Patent Publication No. 2000-162026). In the case of the electronic balance, a second Roberval mechanism orthogonal to a Roberval mechanism (first Roberval mechanism) in which a weighing dish is supported by a movable pillar when viewed from above is used and the movable pillar of the second Roberval mechanism is integrated with that of the first Roberval mechanism. Then, an axial-directional (longitudinal-directional) one-sided load of the first Roberval mechanism is shouldered by the first Roberval mechanism and a one-sided load in the crosswise direction of the first Roberval mechanism (torsional direction about the axis of the first Roberval mechanism) is mainly shouldered by the second Roberval mechanism. Thereby, the adjustment operability is improved by separating the adjustment of a one-sided load in the crosswise direction from that in the longitudinal direction and simultaneously, weakness of the first Roberval mechanism in crosswise-directional rigidity can be covered.
Furthermore, it is preferable to provide a flexible portion soft in a tilted direction of the movable pillar of the first Roberval mechanism, in which a tilt is caused by a one-sided load in the axial direction of the first Roberval mechanism, in other words, a flexible portion for providing flexibility in the longitudinal direction of a beam of the first Roberval mechanism for the second Roberval mechanism. In this case, when a one-sided load in the axial direction of the first Roberval mechanism works and thereby, a force for tilting the movable pillar in the axial direction works, it is possible to absorb the force by the flexible portion and prevent the second Roberval mechanism substantially sharing the movable pillar from being influenced.
Moreover, in the case of the above proposed configuration, by housing the first Roberval mechanism in a square pipe so that the axial direction is parallel with the axis-center direction of the square pipe and setting the second Roberval mechanism to an end face of the square pipe, the positional relation between fixed pillars of two Roberval mechanism is not fluctuated due to a load to be measured and a compact and high-performance electronic balance having a high rigidity is obtained by utilizing the high torsional rigidity of the square pipe.
According to the above proposal of the present inventor, it is possible to substantially adjust one-sided errors independently in the longitudinal and crosswise directions to a weighing dish in the first and second Roberval mechanisms and realize a high-rigidity and compact electronic balance by combining these first and second Roberval mechanism with square pipes. However, according to further detailed examination by the present inventor, the following problems have been clarified.
In other words, when a one-sided load works in the crosswise direction, that is, when a torsional-directional force about the axis of the first Roberval mechanism works, a movable pillar is effective because the second Roberval mechanism is present and thereby, the pillar becomes rigid but the first Roberval mechanism is also slightly deflected. As a result, it is found that a one-sided error by the second Roberval mechanism interferes with a one-sided error by the first Roberval mechanism and this serves a factor for preventing complete one-sided-error adjustment.
The invention is made to solve the above problems, and its objective is to present an electronic balance eliminating above prevention factor and adjusting a one-sided error easily with high accuracy.
To achieve the above objective, an electronic balance of the present invention has a first Roberval mechanism in which a fixed pillar is supported by a movable pillar through an upper and a lower beams parallel with each other to support a weighing dish by the movable pillar and transmit a load working on the weighing dish to an electrical load detecting section through the movable pillar. Further, the electronic balance has a second Roberval mechanism provided with an upper and a lower beams almost orthogonal to the beams of the above first Roberval mechanism and parallel with each other. The movable pillar of the second Roberval mechanism is integrated with that of the first Roberval mechanism. Furthermore, the first Roberval mechanism is housed in a square pipe along its axial direction and the second Roberval mechanism is set to an end of the square pipe. In addition, a flexible portion weak in torsion about the axis of the first Roberval mechanism and rigid in axial-directional one-sided load is formed on the first Roberval mechanism and a flexible portion flexible in tilted direction of the first Roberval mechanism due to a one-sided load in the axial direction of the first Roberval mechanism is formed on the second Roberval mechanism.
The present invention eliminates the above prevention factor in order to more accurately adjust a one-sided error of an electronic balance according to the proposal of the present inventor. According to the proposal of the present inventor, a high-rigidity and compact mechanism is obtained which independently adjusts one-sided errors in the longitudinal and crosswise directions and utilizes the rigidity of a square pipe by using a second Roberval mechanism orthogonal when viewed from above in addition to a first Roberval mechanism and sharing a movable pillar by the both mechanisms. However, in the case of the present invention, a flexible portion weak in torsion about the axis of a first Roberval mechanism and rigid in axial-directional one-sided load is formed on the first Roberval mechanism. Thereby, when a one-sided load causing a torsional moment about the axis of the first Roberval mechanism, that is, a one-sided load in the crosswise direction works, the load is absorbed by the flexible portion formed on the first Roberval mechanism and thereby, the first Roberval mechanism is not deflected. Thus, it is possible to prevent the load from influencing the first Roberval mechanism.
Therefore, according to the present invention, it is possible to make the first and second Roberval mechanisms separately shoulder one-sided loads in the axial direction of the first Roberval mechanism and the direction orthogonal to the axial direction, that is, one-sided loads in the longitudinal and crosswise directions while the mechanisms are hardly influenced each other and easily adjust a one-sided error at a high accuracy by a high-rigidity and compact electronic balance.